1. Field of the Invention
The present invention is in the field of reducing molten metals and/or their slags by top-blowing a reducing stream onto a molten metal bath being treated, and adding finely divided carbonaceous particles about the perimeter of the blow impression created by the reduction gas stream to prevent reoxidation and improve the efficiency of the reduction.
2. Description of the Prior Art
Methods for the continuous or discontinuous reducing treatment of metal melts or of molten slags containing metal oxides utilizing substantially vertical top-blowing of reduction gases for the liberation of the desired metals and/or their compounds by settling and/or volatilization are known. Thus, for example, the German AS No. 26 45 585 (now U.S. Pat. No. 4,210,441, issued July 1, 1980, and whose disclosure is incorporated herein by reference) discloses a system in which reduction gases are blown roughly perpendicularly in the form of focused, high energy gas streams onto the surface of an oxidic slag melt with such a great jet force that a blow impression occurs on the melt surface under every top-blown stream. The blow impression has an approximately toroidal rotating laminar flow of the melt and provides a reaction unit with controlled mass transfer between the blow impression of the melt and the top-blowing stream. High mass transfer rates and reaction kinetic efficiency are improved in comparison with other methods in the region of the blow impression in the melt surface generated by the reduction gas stream.
In such top-blowing of reducing gases onto melts, however, the desired reducing effect is only achieved at the top-blowing location itself and in a tightly limited region adjacent thereto. This region is defined essentially by the blow impression which the gas stream produces in the molten melt. Due to chemical reaction with the melt, the reduction gas has its reduction agent consumed, and its reduction potential decreased as it spreads in the furnace cavity. The ratio of the immediate influencing area of the gas onto the melt to the total melt exposed to the gas space theoretically amounts to at least 1 to 1.4 but as a practical matter amounts to 1 to 3, 1 to 5, or even more.
The used reduction gas together with the gas flowing in due to unavoidable leaking of air into the furnace acts on a very substantial part of the melt to be treated with a weak reduction effect. As a result, a reoxidation of the melt is practically unavoidable on the larger surface presented to the gas space of the treatment furnace in comparison to the reduction effect within the blow impression produced by the gas stream. As a result, a considerable part of the reduction exerted in the blow impression with a high use of reducing agents is lost, due to oxidation processes occurring outside of the blow impression and due to reoxidized melt being agitated back into the blow impression, caused by pulsation of the blowing stream.
The processes cannot be significantly influenced by the material flow within the furnace vessel. At a mass flow of velocity of 1 to 3 m per hour, the rate thereof is 2 to 3 powers of ten lower than the speed of the melt circulating or rotating toroidally in the region of the blow impression.
The reoxidation of the melt beyond the blow impression due to the used gas in the furnace cavity must thus be considered a considerable problem in the successful top-blowing of reduction agents onto a melt. This inadequacy affects all prior proposals for top-blowing of reducing gases or other reduction gases onto melts. It is therefore irrelevant whether, for example, methane, propane, light crude, pyrite or coal are used as reducing agents or as basic materials. In the reducing treatment of melts by means of top-blowing lances, the combustion condition, i.e., the ratio of reducing agent to oxygen, was heretofore viewed as the sole control quantity for the reduction effect or for the reduction potential. This point of view predominated in previous efforts involving top-blowing technology.